BOLERO-5: a phase II study of everolimus and exemestane combination in Chinese post-menopausal women with ER + /HER2- advanced breast cancer

Background The global BOLERO-2 trial established the efficacy and safety of combination everolimus (EVE) and exemestane (EXE) in the treatment of estrogen receptor positive (ER +), HER2-, advanced breast cancer (ABC). BOLERO-5 investigated this combination in a Chinese population (NCT03312738). Methods BOLERO-5 is a randomized, double-blind, multicenter, placebo controlled, phase II trial comparing EVE (10 mg/day) or placebo (PBO) in combination with EXE (25 mg/day). The primary endpoint was progression-free survival (PFS) per investigator assessment. Secondary endpoints included PFS per blinded independent review committee (BIRC), overall survival (OS), overall response rate (ORR), clinical benefit rate (CBR), pharmacokinetics, and safety. Results A total of 159 patients were randomized to EVE + EXE (n = 80) or PBO + EXE (n = 79). By investigator assessment, treatment with EVE + EXE prolonged median PFS by 5.4 months (HR 0.52; 90% CI 0.38, 0.71), from 2.0 months (PBO + EXE; 90% CI 1.9, 3.6) to 7.4 months (EVE + EXE; 90% CI 5.5, 9.0). Similar results were observed following assessment by BIRC, with median PFS prolonged by 4.3 months. Treatment with EVE + EXE was also associated with improvements in ORR and CBR. No new safety signals were identified in BOLERO-5, with the incidence of adverse events in Chinese patients consistent with the safety profile of both drugs. Conclusion The efficacy and safety results of BOLERO-5 validate the findings from BOLERO-2, and further support the use of EVE + EXE in Chinese post-menopausal women with ER + , HER2- ABC. NCT03312738, registered 18 October 2017. Supplementary Information The online version contains supplementary material available at 10.1007/s12672-024-01027-8.


Introduction
Breast cancer is the most common malignancy in women and one of the leading causes of cancer deaths worldwide [1].The incidence of breast cancer in China has increased rapidly over the last number of decades, and currently increases by 3-5% annually [2,3].Despite evidence suggesting that China demonstrates a lower mortality rate compared to other countries, the size of the population and the current increase in breast cancer incidence reinforce the need for safe and effective treatment options [2].
Approximately 70% of breast cancers are estrogen receptor positive (ER +) and are treated with endocrine therapy (such as nonsteroidal aromatase inhibitors [NSAIs, anastrozole or letrozole] or steroidal aromatase inhibitors [exemestane, EXE]) in combination with CDK4/6 inhibitors in the first-line setting [4][5][6].Although these treatments are effective, both de novo and acquired resistance are common, with the majority of patients receiving endocrine therapy developing progressive disease [7,8].Many patients retain sensitivity to hormonal agents, even upon disease progression, but sequential endocrine monotherapy achieves only moderate clinical benefits [8,9].As such, combination approaches with mammalian target of rapamycin (mTOR) inhibitors have been developed as a second-line treatment option [8,10].The PI3K/Akt/mTOR pathway plays a central role in breast cancer cell proliferation and progression [11,12], providing a strong rationale for combining endocrine therapy with mTOR inhibition [13].The PI3K/Akt/mTOR pathway is also implicated in the development of resistance to endocrine therapy, further supporting the rationale for mTOR inhibition in the treatment of ER + breast cancer [14,15].Everolimus (EVE), an mTOR inhibitor, has demonstrated efficacy in enhancing the effects of endocrine therapy in both pre-clinical models and clinical trials in breast cancer [12,16,17].
The global, pivotal, phase III Breast Cancer Trials of OraL EveROlimus-2 (BOLERO-2) trial investigated a combination strategy of EVE and EXE for the treatment of postmenopausal women with locally advanced/metastatic, hormone receptor positive (HR +) disease progressing after anastrozole or letrozole [18].This study established the efficacy and safety of the EVE and EXE treatment combination, with significantly improved progression-free survival (PFS) observed in patients treated with EVE + EXE versus placebo (PBO) + EXE, and no serious toxicity reported [14,18].By investigator assessment, treatment with EVE + EXE prolonged PFS from 3.2 months to 7.8 months (HR, 0.45; 95% CI 0.38, 0.54; p < 0.0001) [18].Similar results were observed following central assessment, with PFS prolonged from 4.1 months to 11.0 months (HR, 0.38; 95% CI 0.31, 0.48; p < 0.0001) [18].
The results of the BOLERO-2 trial led the FDA and EMA to approve combination EVE + EXE for the treatment of HR + , HER-2-advanced breast cancer (ABC) in 2012.This treatment combination is also recommended by both the National Comprehensive Cancer Network (NCCN) and the ESO-ESMO (European School of Oncology-European Society for Medical Oncology) guidelines for postmenopausal women with HR + , HER2-ABC with progressive disease on NSAIs [4,5].
Ethnicity is well-known to impact treatment efficacy and safety, supporting the opinion that potential inter-ethnic differences in anti-cancer drug effect should be considered [19,20].A post-hoc analysis of the BOLERO-2 study was therefore conducted to assess the combination of EVE + EXE in Asian versus non-Asian patients [21].The results of these analyses were consistent with the primary results of BOLERO-2, which demonstrated improved PFS with EVE + EXE in both Asian and non-Asian patients, with no significant differences in the incidence of most adverse events (AEs) observed between Asian and non-Asian patients.However, some AEs such as stomatitis, nasopharyngitis, pneumonitis, and interstitial lung disease were reported more frequently in Asian patients [21].Combination EVE + EXE provided substantial clinical benefit in both an Asian and non-Asian patient population, representing an improvement in the management of postmenopausal women with HR + , HER2-ABC progressing on NSAIs, regardless of ethnicity [18,21].
The China-specific BOLERO-5 study aimed to confirm the efficacy and safety of combination treatment with EVE + EXE (seen in BOLERO-2) in postmenopausal Chinese women with locally advanced/metastatic, ER + , HER2disease following recurrence or progression on anastrozole or letrozole.

Study design and participants
BOLERO-5 is a double-blind, randomized, phase II study evaluating combination EVE + EXE in a Chinese population of postmenopausal women, and was conducted at 15 clinical sites across China.Patients were randomized 1:1 to

Endpoints
The primary endpoint of the study was PFS by local assessment (per RECIST 1.1 criteria), with PFS per blinded independent review committee (BIRC) as a secondary endpoint.PFS was defined as time from the date of randomization to the date of first documented progression or death due to any cause, whichever occurred first.Overall survival (OS), overall response rate (ORR), clinical benefit rate (CBR), time to response, and duration of response (DOR) were all secondary endpoints, and were assessed both locally and by BIRC.Secondary endpoints also included time to deterioration in Eastern Cooperative Oncology Group (ECOG) performance status, AEs, and laboratory abnormalities.The pharmacokinetics of combination EVE + EXE were also assessed.Tumor assessments will be performed every 2 cycles after randomization date until disease progression.The frequency of tumor assessment will be changed to every 12 weeks after primary analysis data cut-off and as clinically indicated, until disease progression.

Statistical analysis
The primary analysis was planned when approximately 110 PFS events were documented, based on local assessment.
As this is a bridging study, an estimation strategy was used over formal hypothesis testing to estimate sample size.
Approximately 160 patients were estimated as being needed for randomization to the two treatment arms to observe the 110 PFS events at approximately 22 months post-randomization of the first study patient.PFS was estimated by Kaplan-Meier analysis using the intention-to-treat (ITT) principle, with median PFS and 90% confidence interval (CI) presented by treatment arm.A Cox regression model (stratified by randomization stratification factors) was used to estimate the hazard ratio (HR) and associated 90% CI for PFS, OS, ORR, and CBR.No hypothesis testing was performed as this was an estimation-based approach.Baseline demographic and disease characteristics data were listed and summarized descriptively by treatment arm.Duration of study treatment exposure, dose intensity, and safety were also summarized by treatment.

Role of the funding source
Novartis Pharmaceuticals Corporation sponsored the study, designed the study and analyzed the data.All authors had full access to the study data and had the final responsibility for the decision to submit this manuscript for publication.The corresponding author had access to all study data and had final responsibility to submit this manuscript for publication.

Patients
A total of 159 postmenopausal women were enrolled across multiple centers in China between 15 September 2017 and 31 March 2020 and were randomized to receive either EVE + EXE (n = 80) or PBO + EXE (n = 79) (Fig. 2).Baseline demographic characteristics were well matched between treatment arms (Table 1); median age (range) at baseline was 56.0 years (36-81) and most patients had an ECOG performance score of 1 (EVE + EXE, 61.3%; PBO + EXE, 68.4%).Baseline disease characteristics were also similar between arms, with most patients initiating study treatment within 3 months of last disease progression (EVE + EXE, 91.3%; PBO + EXE, 97.5%).All patients had stage IV disease at study entry and baseline tumor burden was similar between treatment arms.Overall, 71.1% of patients had visceral involvement and 44.7% of patients had bone metastases; most patients had metastases in other sites besides the central nervous system (CNS), bone, lung, liver, and visceral sites (EVE + EXE, 75.0%;PBO + EXE, 81.0%) (Table 1).
A total of 116 PFS events occurred by the data cutoff of 19 May 2020, with a median follow-up of 14.5 months.Most patients (n = 138; 86.8%) had discontinued randomized treatment by data cutoff, with disease progression the primary reason for discontinuation (EVE + EXE, 55.0%; PBO + EXE, 79.7%).At data cutoff, median duration (range) of EVE therapy in the EVE + EXE arm was 16.1 weeks (2.4-95.0),while median duration (range) of EXE therapy was 17.1 weeks (range: 2.4-95.1).For the PBO + EXE arm, median duration of EXE therapy was 8.4 weeks (range: 2.1-96.1); the same median duration and range were observed for PBO therapy.Both treatment arms achieved a median relative dose intensity of 100%, with a median dose intensity of 10 mg/day for PBO and EVE, and 25 mg/day for EXE.Mean (SD) dose intensity in the EVE + EXE arm was 9.1 mg/day (1.45) for EXE and 25.0 mg/day (0.00) for EVE, whereas in the PBO + EXE arm it was 10.0 mg/day (0.14) for PBO and 25.0 mg/day (0.00) for EXE.In the EVE + EXE arm, 23 patients (28.8%) were exposed to EVE and 28 patients (35.0%) were exposed to EXE for a period of ≥ 32 weeks; in the PBO + EXE arm, just 13 patients (16.5%) received treatment for ≥ 32 weeks.
Most patients (62.5%) in the EXE + EVE arm had at least one dose adjustment (reduction and/or temporary interruption) of EVE, while 17.7% of patients in the PBO + EXE arm had at least one dose adjustment of PBO.At least one EVE dose reduction was required in 32.5% of patients in the EVE + EXE arm compared to 2.5% of patients with at least one reduction of PBO in the PBO + EXE arm.At least one dose interruption in EVE was required in 56.3% of patients in the EVE + EXE arm versus 17.7% of patients with at least one interruption in PBO in the PBO + EXE arm.In the EVE + EXE arm, the primary reason for EVE dose adjustments or interruptions was AEs (responsible for 21.3% of reductions and 47.5% of interruptions).More patients had at least one dose interruption of EXE in the EVE + EXE arm (40.0%) vs the PBO + EXE arm (16.5%); no patient in either arm required an EXE dose reduction.

Time to response, duration of response and ECOG performance status
Analyses of the Kaplan-Meier estimate of the time to response were not performed due to the low number of responses in both arms (8 partial responses per investigator assessment; one complete response and 8 partial responses per BIRC assessment).Time to response and duration of response data are shown in Table S1.Deterioration in ECOG performance status by ≥ 1 point was noted in few patients (8/80) in the EVE + EXE arm and in no patients in the PBO + EXE arm (Figure S1). ).The incidence of treatment-related adverse events (TRAEs) in the BOLERO-5 study was consistent with the known safety profile of both drugs, with no new safety signals identified in Chinese patients (Table 3).Most TRAEs were Grade 1 or Grade 2, but the incidence of Grade 3 TRAEs was higher in the EVE + EXE arm (45.0%) compared to the PBO + EXE arm (11.4%).The most common Grade ≥ 3 TRAEs in the EVE + EXE arm versus the PBO + EXE arm were hyperglycemia (10.0%versus 0%), hypokalemia (6.3% versus 0%), hypophosphatemia (6.3% versus 0%), stomatitis (7.5% versus 1.3%) and pneumonia (5% versus 0).The most common adverse events of special interest (AESIs) all occurred more frequently in the EVE + EXE arm (Table S2).These included stomatitis (reported in 73.8% of patients in the EVE + EXE arm versus 11.4% of patients in the  No unexpected findings were reported in relation to clinical laboratory or vital signs.Hematological abnormalities were more commonly observed in the EVE + EXE arm than in the PBO + EXE arm.Decreased hemoglobin was the most common abnormality (EVE + EXE, 57.5%; PBO + EXE, 24.1%), followed by decreased absolute lymphocyte count (EVE + EXE, 45.0%; PBO + EXE, 22.8%).Clinical abnormalities were also more frequent in patients treated with EVE + EXE versus PBO + EXE, and included increased triglycerides (63.8% versus 25.3%) and increased cholesterol (83.8% versus 29.1%).
The incidence of AEs leading to permanent study discontinuation was low in both treatment arms (EVE + EXE, 11.3%; PBO + EXE, 3.8%).The most frequent AEs resulting in discontinuation were pneumonia (n = 3 in the EVE + EXE arm) and bilirubin increase (n = 2 in the EVE + EXE arm).Serious adverse events (SAEs) were reported more frequently in the EVE + EXE arm vs the PBO + EXE arm (20% versus 12.7% reporting ≥ one SAE).The most common SAEs in the EVE + EXE arm were pneumonia (7.5%) and interstitial lung disease (2.5%).A total of five deaths occurred during the study treatment (EVE + EXE, n = 3; PBO + EXE, n = 2), all of which were attributed to the underlying malignancy.

Discussion
The BOLERO-5 study provides strong evidence for the efficacy and safety of EVE + EXE in Chinese post-menopausal women with ER + , HER2-, locally advanced, recurrent, or metastatic breast cancer following recurrence or progression on prior letrozole or anastrozole.The global BOLERO-2 study established the efficacy and safety of combination of EVE + EXE [14,18] and led to the approval of EVE for the treatment of post-menopausal women with HR + , HER2-ABC.The Asian sub-population analysis of BOLERO-2 (N = 143), as well as the recent Phase IIIB EVEREXES trial of this treatment combination in Asian patients (N = 199), support the use of combination treatment with EXE + EVE to improve PFS without compromising quality of life (QoL) [18,[21][22][23], representing a significant advancement in the management of ABC.Following the publication of the BOLERO-2 results, several international guidelines (including the NCCN and ESO-ESMO ABC5 treatment guidelines) recommended combination treatment with EVE + EXE for this population [4,5].
In BOLERO-5, combination EVE + EXE achieved remarkably similar PFS to that observed in the global BOLERO-2 trial (median PFS per investigator assessment: 7.4 months versus 7.8 months, respectively) and provided clinically meaningful benefits for the Chinese patients enrolled [14,18,21].Treatment with EVE + EXE yielded a 48% risk reduction in PFS (per investigator assessment) and prolonged median PFS by 5.4 months, from 2.0 months (PBO + EXE) to 7.4 months (EVE + EXE).The PFS rate at 12 months was also numerically greater in the EVE + EXE arm compared to the PBO + EXE arm, indicating a more sustained treatment benefit with this combination.The magnitude of this PFS benefit compares favorably with other studies of everolimus and anti-estrogen therapy in this setting [16,17], and also with the alternative treatment options available for these patients [8,9,24].Patients receiving combination treatment with EVE + EXE also achieved greater ORR and CBR, further supporting the primary PFS outcome.Similar to PFS data, the observed ORR for EVE + EXE in BOLERO-5 was very similar to that reported in the BOLERO-2 global trial (8.8% versus 9.5%, respectively).The similarity of results between investigator and BIRC assessment of primary and secondary outcomes lends further credence to the robust nature of these data.While the OS data were immature at the primary cutoff, long-term follow up will enable further evaluation of the benefits of EVE + EXE combination treatment.The publication of the pharmacokinetic results of this trial will also provide an opportunity to evaluate the impact of combination EVE + EXE on estrogen concentration.
The combination of EVE + EXE was well tolerated in this Chinese patient population, with the incidence of AEs consistent with that previously reported with EVE and other rapamycin analogues [25], as well as with the safety profile of this treatment combination in HR + , HER2-ABC [14,18,21].While the incidence of AEs was increased in the EVE + EXE group versus PBO + EXE, analysis of patient-reported outcomes in a similar patient population in the BOLERO-2 study revealed that QoL was maintained with this treatment combination, despite the increase in AEs [22].Thus, the clinical efficacy of combination EVE + EXE may outweigh the potential toxicity, particularly when the impact on patient QoL is negligible.Most AEs reported in this study were Grade 1 or 2 in severity and could be managed with appropriate dose adjustments (reduction and/or interruption) and supportive therapies.The low discontinuation rate due to AEs in the EVE + EXE arm indicates that most AEs were well managed, allowing patients to continue the treatment as long as it provided clinical benefit.https://doi.org/10.1007/s12672-024-01027-8Some differences in AESIs were reported between BOLERO-5 and BOLERO-2, although these differences were < 15%.These were for stomatitis (73.8% versus 68.0%), hemorrhages (16.3% versus 30.3%), and non-infectious pneumonitis (23.8% versus 21.8%) [26].These events can all be effectively managed in this setting using clinically-defined management strategies [27][28][29].A higher incidence of hyperglycemia and lower incidence of stomatitis were observed in BOLERO-5 compared with the BOLERO-2 Asian population.Stomatitis was reported in 73.8% of patients administered EVE + EXE in BOLERO-5 compared to 84.7% of patients in the Asian sub-population of BOLERO-2 [26].It is reassuring that the incidence of stomatitis was lower in BOLERO-5, which can likely be attributed to the implementation of prophylactic treatment with corticosteroid-based mouthwash and to the improved management of this event over time [30].However, no data on patient adherence to mouthwash usage were collected.The higher incidence of hyperglycemia in BOLERO-5 versus the Asian patients of BOLERO-2 (51.3% versus 11.2%) can possibly be explained by the evolution in case retrieval strategies used for mapping preferred terms to AESI [26].The BOLERO-2 Asian subgroup analysis case retrieval strategy included hyperglycemia, diabetes mellitus, blood glucose increased, glycosuria, type 2 diabetes mellitus, and glucose urine present under the AESI of hyperglycemia/new onset diabetes mellitus, while the BOLERO-5 case retrieval strategy included hyperglycemia, diabetes mellitus, blood glucose increased, diabetic ketosis, glucose tolerance impaired, and glycosylated hemoglobin increased [26].Of note, starting treatment with EVE at 5 mg/day rather than 10 mg/day may increase treatment compliance due to good tolerability and similar effectiveness [31].
The efficacy of combination EVE + EXE observed in BOLERO-5 was remarkably similar to the global BOLERO-2 trial, particularly the results of the BOLERO-2 Asian sub-population analysis [18,21].The safety profile of this treatment combination in a Chinese population was also largely consistent with the safety profile observed in the global BOLERO-2 trial [18,21].Similar efficacy results were also observed in the EVEREXES trial of combination EVE + EXE conducted in Asia, providing further support for this treatment combination in Chinese patients [23].
Treatment guidelines currently recommend the use of endocrine therapy in combination with CDK4/6 inhibitors in the first-line setting [4][5][6].This study was carried out before this recommendation was implemented, and as such-as in the BOLERO-2 study [18]-no enrolled patients had received CDK4/6 inhibitors prior to the study start.No clinical trials have been carried out to date assessing the efficacy of EVE + EXE in patients pretreated with CDK4/6 inhibitors; however, a retrospective analysis of 622 patients with HR +, HER2-metastatic breast cancer showed that EVE + EXE was effective in patients who had received endocrine therapy alone and in combination with CDK4/6 inhibitors [32].Similar results showing no impact of previous treatment with CDK4/6 inhibitors in OS were obtained in two smaller retrospective studies [33,34].
This study is not without its limitations, and these results should be interpreted with a degree of caution.The small sample sizes in each group, in particular the small number of patients classed as responders, limit the interpretation of these data.Secondly, no QoL data were collected during this study, so the impact of AEs on the QoL of the Chinese postmenopausal women enrolled in this trial is unknown.
In conclusion, treatment with EVE + EXE provided a clinically meaningful PFS benefit in this heavily pre-treated population of Chinese patients with ER + , HER2-breast cancer, further validating the clinical benefit reported in the global BOLERO-2 trial.Combination treatment with EVE + EXE thus offers a significant improvement in the management of Chinese patients with ER + , HER2-breast cancer.

Table 2
Overall response rate and clinical benefit rate ORR is defined as the proportion of patients with the best overall response of completed response or partial response per RECIST 1.1.CBR is defined as the proportion of patients with best overall response of CR, PR, or overall lesion response of stable disease (SD for measurable disease and non-CR/non-PD for nonmeasurable disease) ≥ 24 weeks.DOR is defined as the time from first documented response (PR or CR) to the date of first documented disease progression or death due to any cause CBR, clinical benefit rate; CI, confidence interval; CR, complete response; DOR, duration of response; EVE, everolimus; EXE, exemestane; PBO, placebo; PD, progressive disease; PR, partial response; RECIST, Response Evaluation Criteria In Solid Tumors; SD, stable disease

Table 3
Treatment-related adverse events by preferred term (occurring in ≥ 20% of patients in either treatment arm)A patient with multiple severity grades for an AE is only counted under the maximum grade.Adverse events occurring more than 30 days after the discontinuation of study treatment are not summarized.Adverse events were described as per MedDRA version 23.0, CTCAE version 4.03 EXE arm), cytopenia (53.8% versus 24.1%), hyperglycemia/new onset diabetes mellitus (51.3% versus 8.9%), dyslipidemia (47.5% versus 7.6%), and severe infections (45.0%versus 17.7%).Non-infectious pneumonitis, frequently associated with mTOR inhibitors, was reported in 19 patients (23.8%) randomized to EVE + EXE and all cases were considered to be treatment-related.The majority of cases were mild in nature, with only 1 case (1.3%) of Grade ≥ 3 non-infectious pneumonitis reported.No patients in the PBO + EXE arm experienced non-infectious pneumonitis.
* AE occurs in a greater proportion of patients randomized to everolimus plus exemestane (≥ 15% difference versus placebo plus exemes-